1. Field of the Invention
The present invention relates generally to a motor actuation control device, and particularly to a motor actuation control device which is capable of steadily controlling a motor even during rapid changes in the rotation rate of the rotor.
2. Description of the Related Art
In most motor vehicles, including electric vehicles and hybrid vehicles, a direct current power source such as a battery is provided, and an alternating current motor is provided as a power source. In addition, an inverter is provided between the direct current power source and the alternating current motor to convert the direct current power to the alternating current power.
PMW (pulse width modulation) control is a known technique for controlling the alternating current motor via inverters. PWM control is one type of voltage conversion control techniques for voltage type inverters, in which a pulse signal called a PWM signal is supplied to a switching element provided in the inverter to control on/off timing of the switching element. By adjusting the on/off timing of the switching element, it is possible to control the voltage applied to the motor.
PWM signals are generated through a triangle wave comparison method. Specifically, a command signal which determines a voltage value to be applied to the motor is compared to a voltage value of a triangle wave which is also called a carrier wave to generate a PWM signal.
The voltage value of the command signal is determined continuously based on a rotor electrical angle and a torque requirement value. The command signal generally has a sinusoidal waveform and the cycle of the command signal is increased or decreased according to changes of the torque requirement value and the rotation rate of the rotor when the synchronous motor, such as a permanent magnet motor, is used.
The triangle wave is generated through integration of clock signals. The frequency of the clock signals are set by a control unit or the like provided in the motor vehicle.
In generating the PWM signal, the number of pulses of the PWM signal provided during one cycle of the command signal is determined by the ratio of the frequency of the triangle wave to the frequency of the command signal. For example, if the ratio of the frequency of the triangle wave to the frequency of the command signal is 15, then fifteen pulses are provided in the PWM signal during one cycle of the command signal.
On the other hand, if the inverter turns on and off a great number of times within a short period of time, switching loss occurs and the switching element may as a result become overheated, which may lead to performance errors of the element. Measures to avoid overheating of the switching element, such as providing more than one switching element to disperse heat and prevent overheating, have been attempted, but, in order to reduce costs and for other reasons, modern inverters include fewer switching elements compared than earlier conventional inverters. Because the preventive measure noted above cannot be applied to such inverters, the number of pulses of the PWM signal during one cycle of the command signal must be set to a relatively small number.
However, when the number of pulses of the PWM signal during one cycle period of the command signal is set to a relatively small number, it is necessary to change the frequency of the clock signal in response to changes of the frequency of the command signal and maintain the number of pulses of the PWM signal during one cycle period of the command for the sake of the stable PWM control.
For example, if it is desired to increase the rotation rate of the rotor, the frequency of the command signal is increased as well. In contrast, if the frequency of the clock signal is fixed, the number of pulses of the PWM single during one cycle of the command signal is decreased.
If a relatively large number of pulses (e.g., 15-20 pulses) are included in the PWM signal during one cycle of the command signal, the influence of the decrease in the number of pulses may be small. On the other hand, however, if the number of pulses included in the PWM signal during one cycle of the command signal is relatively small (e.g., 5-10 pulses), the pulse number decrease gives a greater influence. In this case, as the number of pulses of the PWM signal during one cycle of the command signal is decreased, the inverter is not able to output an expected voltage as designated by the command signal. Eventually, a control failure, such as overload of the inverter, detuning of the motor, or the like, may occur
To deal with this problem, a control method has been known in which the frequency of the triangle wave is changed before rapid changes in the rotation rate (rpm) of the rotor and a change of the number of pulses of the PWM signal, while the number of pulses of the PWM signal is maintained at a fixed value. For example, Japanese Patent Laid-Open Publication No. 2007-159367 (Patent Document 1) discloses a technique in which the frequency of the triangle wave is raised when the increasing ratio of the rotation rate of the rotor exceeds a threshold value.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-159367
The rotation rate of the rotor is not calculated until after the rotor is rotated to some extent. Usually, the rotation rate of the rotor is calculated from the change of the electrical angle of the rotor during a predetermined time period while considering external factors, such as changes in the torque requirement value, the running resistance, etc. Because performing such calculation requires about 1 to 3 milliseconds, the frequency of the clock signal is adjusted about every 1 to 3 milliseconds.
On the other hand, the rotor electrical angle continuously increases and decreases in proportion to the rotation of the motor, particularly, for example, when road conditions cause tire slip to occur in a motor vehicle. This may cause the rotor electrical angle to change rapidly. In response, the frequency of the command signal may also change rapidly in less than 1 to 3 milliseconds, thereby increasing or decreasing the number of pulses of the PWM signal during one rotation of the rotor.
In consideration of the above, an object of the present invention is to enable rapid response to changes in the rotation of the motor when PWM control is performed.